1. Field of the Invention
The present invention relates to a magnetic head, and more particularly, to a magnetic head and a recording apparatus that attains high-density recording.
2. Description of Related Art
As a result of increasing density of magnetic recording apparatus in recent years, GMR heads using a spin valve film having a magnetoresistive effect for a sensor film are used as a read head. These heads also use a permanent magnet layer abutted junction type for magnetic domain control. And thus, the narrowing of the read track width has been achieved. FIG. 2 shows a structure of a magnetic head of a permanent magnet layer abutted junction type of a prior art. In this structure, a GMR sensor film 1, permanent magnet layers 2 and electrode films 4 are interposed between two magnetic shields, namely, a lower magnetic shield S1 and an upper magnetic shield S2. The permanent magnet layers 2 are respectively located adjacent to the ends of the GMR sensor film 1, and the electrode films 4 are respectively located just above the permanent magnet layers 2. The permanent magnet layer 2 and the electrode film 4 have a role as the electrode for supplying current to the GMR film. The permanent magnet layer 2 has a role of magnetic domain control of providing a magnetic field to a free layer 30 constituting the GMR film 1, thereby forming a single magnetic domain. Since the magnetic field from the permanent magnet layer 2 is larger as it goes nearer to the permanent magnet layer 2, magnetizing rotation of the free layer is restricted by the magnetic field in the minute region of the GMR film near the permanent magnet layer to result in a region of low sensor sensitivity. The region is hereinafter referred to as “low sensitivity region”. The sensitivity distribution of the sensor is shown in the figure in which skirt regions on both sides of the hill-like sensitivity distribution each represent the “low sensitivity region” 6. The low sensitivity region has a width of about 0.05 to 0.1 μm from the end of the permanent magnet layer. In a case where the read track width is as large as about 1 μm, the ratio of the low sensitivity region accounts for about 20% of the read track width, which causes no substantial problem. However, as the read track width is narrowed, the ratio of the low sensitivity region to the read track increases to abruptly decrease the read output.
FIG. 3 shows the dependence of the read output on the read track width in a case where the sense current is made constant at a constant MR height. As the track width decreases, the read output decreases abruptly relative to the proportional relation shown by a dotted line in the graph, and the output is reduced to zero at an effective track width of 0.15 μm in extrapolation. At a recording density of 70 Gb/in2 or more, it is necessary that the effective read track width is 0.2 μm or less and, since about 1 mV of the read output is necessary for driving a hard disk drive normally, information written in a recording medium cannot be read by the conventional permanent magnet abutted junction type GMR head because the read output is excessively low.
In order to prevent abrupt lowering in the read output along with narrowing of the read track, an electrode overlap type GMR head has been proposed in the prior art.
FIG. 4 shows the structure of the electrode overlap type GMR head. A pair of permanent magnet layers 2 is provided each of which is adjacent to each side of a GMR sensor film 1 formed to a desired width. A pair of electrode films 4 provided each of which is disposed on each of the permanent magnet layers 2. Each of the electrode films 4 is configured to overlap the GMR film 1, in which the distance DLD between the pair of electrode films 4 is made smaller than the gap DCD between the pair of magnet domain control films 2. In this structure, since a region where current flows mainly in the GMR film is the region DLD between the pair of electrodes in the drawing, a region having the sensitivity of the sensor film is the DLD region in the drawing. Since the permanent magnets are positioned apart from the region sufficiently, it was expected that the lowering of the sensitivity would not be caused in the DLD region.
However, by examining the sensitivity distribution of the sensor closely, it has been found that the sensitivity of the sensor film is distributed more widely than the region denoted by DLD between the electrodes and the read track width is greater than the electrode distance DLD. Accordingly, in order to obtain a desired read track width, it has been found that the inter-electrode film distance DLD has to be less than a desired width. The reason why the read track width is wider than the DLD is that magnetic fluxes of a medium that entered the free layer just below the electrode propagates to the free layer in the DLD region. The propagated magnetic field causes the resistance in the GMR sensor film to change. One of the method for avoiding the problem is to narrow the inter-electrode film distance DLD in prospect of the widened read track width. However, since this needs a photolithographic process technique of preparing narrower inter-electrode film distance DLD, it leads to difficulty in view of the fabrication process.
On the other hand, one of the other ways for improving the sensitivity of the sensor is by lowering of the magnetic domain control force. However, this makes the magnetic domain control insufficient, due to variations in the amount of overlap or variations in the angle of the electrode 4 or the magnetic domain control film (permanent magnet layer 2) at the end of the sensor film and results in waveform instability.
As stated above, in the prior arts, it is a problem that an abrupt lowering of an output occurs, which results from narrowing of a read track, in using conventional permanent magnet abutted junction type. Also, attaining a read head having high read sensitivity and having a narrow read track width with less waveform instability. The problem caused by the narrowing of the track width applicable to high track density includes a problem with the narrowing of the magnetic track width, which particularly results in a significant problem with reading. This is because magnetization at the end of the geometrical track width is less moveable under the effect of the magnetic domain control field and a dead region with no sensitivity is present. That is, the ratio of the dead region increases as the track width is narrowed to make lowering of the read output conspicuous. This results in a particularly significant problem at a track width of 200 nm or less. Therefore, a magnetic head having a lead overlay structure or a magnetic film of high magnetoresistive effect has been known. In the lead overlaid structure, the electrode distance has to be narrower than the width of the free layer. Therefore, it is difficult to use for the narrow tracks, which the width is 200 nm or less, because it is difficult to form it by lithography, or because of the variation in track width due to the variations in the matching of the width of the free layer and the electrode distance.
On the other hand, larger resistance change has been studied by using a magnetic film of a high saturation magnetization such as a CoFe single layered film having a high magnetoresistive ratio. However, since larger magnetic domain control force is necessary as the saturation magnetization is larger, this is not actually so effective in the narrow track. That is, if the magnetic domain control is weakened, higher output can be obtained in accordance with the magnitude of the magnetoresistive ratio. However, weak magnetic domain control also results in increased magnetic instability. Accordingly, the magnetic domain control has to be enhanced. Further, a stacked ferrimagnetic structure for the free layer has also been studied, which is described in both U.S. Pat. No. 5,408,377 and Japanese published patent JPA 2000-113418. In the structure disclosed, the magnetizing direction of the free layer changes easily merely by applying an external magnetic field of low intensity, due to the reduced effective magnetic film thickness. This leads to enhanced sensitivity. However, the stacked ferrimagnetic free layer has to be kept anti-parallel, even when a vertical bias field or a lateral bias field is applied, because of the Joule heat by the sense current, or because of the high temperature of the surroundings. And if anti-parallel can not be kept, even partially, it results in a problem of leading to the disturbance of the track profile. Further, the anti-parallel coupling strength depends highly on the film thickness. The problem is that the thickness control is difficult.
Moreover, descriptions regarding the track width are not found in the above-identified prior arts, and they are merely based on the study of the magnetic film structure and the effect in the narrow track is not considered. Further, U.S. Pat. No. 5,408,377 only studies in an area of a track width of as wide as 400 nm as a result of calculation. However, the ratio of the dead region is small in this area and the problem of the dead region is not studied. That is, the relation between the behavior of magnetization at the track end and the dead region in the magnetic head of a narrow track width is not apparent in view of the studies made so far.
Accordingly, what is needed is a magnetic head that obtains stable retrieving output, which can be used for narrow tracks.